Millimeter-wave measurements in high finesse cavity of nitro-derivatives traces: A new insight in the explosive vapor sensing

Abstract

Cavity-Enhanced Absorption Spectroscopy (CEAS) and Cavity Ring-Down Spectroscopy (CRDS) are well established for sensitive infrared measurements of gas-phase compounds at trace levels using their rovibrational signatures. The recent successful development of a THz Fabry–Perot spectrometer by Hindle et al. (2019) shows that the adaptation of such techniques to submillimeter wavelengths allows to probe rotational transitions of light polar compounds. Here we report on the development of a new millimeter-wave resonator, covering the 150–215 GHz frequency range, and based on a low-loss corrugated waveguide with homemade highly reflective photonic mirrors obtaining a finesse above 3000 at around 164 GHz. With an effective path length of two kilometers, a significant sensitivity has been evaluated, and the detection of semi-volatile organic vapors at a trace level may be now envisaged at room temperature. We applied this technology to detect gas-phase explosive taggants and precursors, confirming a detection limit of 2 ppmv for nitromethane (NM). Leveraging the unique characteristics of the millimeter wave frequency band, we showcase highly selective detection in quasi-realistic environments of complex chemical mixtures involving explosive taggants with close chemical structures such as nitrotoluene isomers. Furthermore, we successfully address the challenge of detecting these nitro-derivative compounds vaporized from model matrices: KCl matrices, granular and plastic (NP91) explosives respectively in conventional and pyrotechnic laboratories. Our findings underscore this approach as a potent tool for practical explosive detection applications.